Which App I need to only measure Third Octave Bands?
You can perform 1/3 octave measurements with SignalScope Advanced or with SignalScope X and an Advanced Tool Set subscription.
You can download SignalScope X and try out any of the subscriptions for free for up to 7 days.
Note that if you need to measure percentile-exceeded sound levels in 1/3 octaves or you need 1/3-octave data logging, then you would need the Pro Tool Set. In that case, if you only need to acquire sound levels (not voltage, current, vibration, etc), then you could save some money by using SoundMeter Pro instead of SignalScope Pro.
Let us know if you have any other questions.